The moon, once an enticing siren for intrepid space travelers, has begun to feel old-hat to some. Even NASA has shifted its sights to fresher targets for exploration: an asteroid and Mars.

But many refuse to give up the goal of sending humans back to the moon. They point to the relative ease of getting there, the deserving science that could be furthered there, and the moon's ability to serve as a testing ground for even more complicated deep space missions.

Space.com asked many scientists, astronauts and space leaders about the role our moon could play in future exploration. Here, then, is the case for the moon.

The moon is still exciting
Just because a dozen humans set foot on a small fraction of lunar surface does not mean humanity has conquered the moon, proponents of revisiting it say. And those 12 emissaries from Earth — NASA's Apollo astronauts — took their walks about 40 years ago. (The six manned moon landings were from 1969 through 1972.)

"There's a special place in people's hearts for the moon," said Eric Anderson, president and chief executive of the Virginia space tourism firm Space Adventures. "We see it every night. It's a symbol of the future of space exploration and of one of the greatest achievements humanity has ever had."

He is not the only one immune to a "been there, done that" attitude.

"It's very exciting to me!" Peter Diamandis, chairman and chief executive of the X Prize Foundation, wrote in an e-mail. "The moon is exciting as Alaska was in the 1800s. It is a frontier where tremendous wealth will be created and resources will be extracted for humanity's expansion beyond the Earth."

Some say that whether the moon is emotionally inspiring as a destination is beside the point.

"If you're dependent on public excitement to determine whether something is worth doing or not, you're not going to do much," said Paul Spudis, a lunar geologist at the Lunar and Planetary Institute in Houston. "The public is mostly concerned with their own lives, rightly so."

Important science
Moreover, there are still many pressing unanswered scientific questions about our nearest space neighbor. [10 Coolest New Moon Discoveries]

"There is a lot of good science that we've only scratched the surface on," said former astronaut Tom Jones, a veteran of four space shuttle flights, who has a doctorate in planetary science.

For instance, in the years since men first walked on the moon, orbiting satellites have revealed evidence that its surface contains hidden reserves of water. Future expeditions could study this water and learn more about the complex makeup of this planetary body.

Science editor Alan Boyle's blog: "Astronaut Abby" is at the controls of a social-media machine that is launching the 15-year-old from Minnesota to Kazakhstan this month for the liftoff of the International Space Station's next crew.

"The Apollo astronauts made only brief visits to only six places on the moon, all near the equator," said Richard Vondrak, deputy director of the Solar System Exploration Division at NASA's Goddard Space Flight Center in Greenbelt, Md. "Our most recent missions, such as LRO and LCROSS, are revealing new secrets of the moon and helping us to identify new places to go, such as the polar regions."

How the moon formed and evolved are still open questions. While many scientists think the moon was created when a Mars-size space rock slammed into Earth and knocked off a piece of it, the details need to be worked out, and other scientists dispute that explanation.

Finally, the current absence of a dynamic surface or active volcanism enables the moon to preserve a history of all its impacts from space rocks. These impacts tell a tale of the history of our solar system — one we can't get on Earth because our planet is always changing and covering over the record of its past.

"One current debate in impact studies is: Is the impact flux on Earth episodic — does it occur in cycles?" Spudis said. "The Earth's dynamic surface erases the impact record. But you can retrace the bombardment history of Earth by studying the bombardment history of the moon."

A mission for humanity
Getting the answers to these questions and others could depend on sending people, rather than just robots, to the lunar surface.

"Astronauts can accomplish scientific exploration that is beyond the capability of robotic explorers," Vondrak said. "Moreover, human missions have flight systems that allow for enhanced capabilities that are not possible in robotic spacecraft.

"For example, the hundreds of kilograms of lunar samples returned by Apollo greatly exceeds the capacity of robotic flight systems. Astronauts can perform geological exploration and complex scientific research far better than robotic systems."

The moon also gets points as an exploration destination because it is a much simpler matter to get there than to, say, Mars.

"You don't have to wait months for launch windows, and it's only a three-day trip," Spudis said.

Stepping stone
For many, the question isn't, "Should we go to the moon OR Mars?" but rather, "Where should we go first?"

"The moon is the perfect ground to get our feet wet for Mars," said space entrepreneur Robert Bigelow, founder of Bigelow Aerospace, which is aiming to put private space station modules in Earth orbit.

In fact, our chances of making it to Mars could ride on whether we go back to the moon first, proponents say.

"The moon is the only logical first destination," said William Pomerantz, senior director of space prizes at the X Prize Foundation. "It only makes sense to use the moon as a testing ground and proving ground and staging ground for missions to Mars and other parts of the solar system."

NASA could practice for the Mars flight by building habitats on the lunar surface and developing techniques there to utilize indigent resources, like water, some argue. That would improve the prospects for a livable and sustainable stay on Mars.

Planetary scientist Carolyn Porco of the Space Science Institute in Boulder, Colo., who leads the imaging team on NASA's Cassini mission to Saturn, said: "I'm worried that if we make a hell-for-leather push to go directly to Mars now, it'll be Apollo all over again. First we'll just squeak by in getting there. Then we'll get there, plant a flag and say we did it, and then everyone will live off that high for 50 years, and the argument will arise that we'll never have to do it again.

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"That's not the kind of approach to human spaceflight that will lead to a permanent human presence beyond the Earth."

By developing the technologies needed to travel back to the moon and set up shop there, humanity can work out the kinks before embarking on a more challenging and lengthy Mars mission.

"Some propose that we concentrate only on Mars. Without the experience of returning to the moon, however, we will not have the engineering, operational, or physiological insight for many decades to either fly to Mars or land there," planetary scientist Harrison "Jack" Schmitt –who as a NASA astronaut was the last person to walk on the moon– wrote in a statement.

Useful lunar resources
Plus, travelers to the moon could mine the lunar surface for commodities useful not just for building lunar colonies but for life on Earth, some say.

"It has resources for exploration, and some we need here on Earth," said Tim Pickens, leader of the Alabama Rocket City Space Pioneers, a team competing for the Google Lunar X Prize.

Pickens cited helium-3, which is hard to find on Earth but thought to be common in the top layers of lunar dirt. This isotope could be useful for nuclear fusion research, and many hope it could be converted into rocket fuel.

That's not all.

"The moon's surface is rich in silicon, aluminum, titanium, and many of the rare earth metals," said science fiction author Ben Bova. "You've got raw materials for solar cells and structures."

He said the silicon on the moon could be mined to build solar panels.

"That's the shape of the future. I think we may see this in the next 10 to 20 years," Bova said.

The recent finding of water on the moon could be a game-changer.

"What I think is the most significant and most interesting about the new discoveries is the discovery of massive amounts of water at the poles of the moon," Spudis said. "It's a life-sustaining consumable."

He said that water could be used for drinking, broken down into hydrogen and oxygen for breathable air, and even converted into rocket fuel.

"The sooner we access these resources," said astronaut Jones, "the sooner we have a beehive of economic activity beyond low-Earth orbit."

Foothold in space
The moon could provide not just practice for future missions elsewhere, but perhaps a midway point on those journeys. Spacecraft could land and refuel there before taking off for farther destinations.

Science fiction writer Orson Scott Card admitted such a project could be a political hard sell, and the endeavor would not be guaranteed to work, but he said: "I strongly believe that we are foolish if we don't establish a moon base and use it as a launch point for expeditions elsewhere in the solar system."

Ultimately, the moon may represent the best chance humanity has of establishing a foothold in space.

"We need to expand the human 'economic sphere' beyond geostationary orbit to the moon," Diamandis said. "The moon represents a critical platform for humanity, [a] foothold into the solar system for the acquisition of water, resources and energy."

And it just might replace Florida as a mecca for senior citizens.

"The moon might make a great retirement center," said Bova, who is 78. "Living at one-sixth gravity might make my aching joints feel a lot better."

Up-close exploration of the moon, Earth's only natural satellite, began in 1959 when the Soviet Union launched its Luna 1 spacecraft on a flyby mission. NASA quickly followed up with missions of its own. Since then, the Europeans, Japan, China and India have launched their own lunar exploration programs. This view shows the moon as seen from the international space station. Click the "Next" arrow above to check out 11 images from the moon made over the last 50 years.
(NASA)
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1959: Far side in full view

In October 1959, the Soviet Union's Luna 3 spacecraft - the third successfully launched to the moon - made history as the first probe to image the far side of the moon. The photos were fixed and dried on the spacecraft and beamed back to Earth. Though fuzzy by today's standards, the images showed stark differences from the near side, including relatively few dark areas, called lunar maria.
(RSA via NASA)
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1966: A restored ‘Earthrise'

In 1966 and 1967, NASA sent a series of Lunar Orbiter spacecraft to collect detailed images of moon's surface in preparation for the Apollo program. The tapes were then put in storage. Decades later, researchers with the Lunar Orbiter Image Recovery Project collected the vintage hardware required to play back the imagery. That imagery was digitized , reproducing the images at a much higher resolution than previously possible. On Nov. 11, 2008, the project researchers released this enhanced photograph of Earth rising above the lunar surface, originally made by Lunar Orbiter 1 in 1966.
(LOIRP / NASA)
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1968: The most famous 'Earthrise'

On Christmas Eve 1968, Apollo 8 astronauts Frank Borman, Jim Lovell and William Anders made history as the first humans to orbit the moon. They were scouting its surface for a suitable landing spot for future missions. But the sight of Earth rising above the moon's horizon caught their - and the world's - attention. The photograph, called "Earthrise," is among the most famous ever made from the moon.
(NASA)
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1969: One small step

On July 20, 1969, an estimated 1 billion people around the world were glued to television screens to watch astronaut Neil Armstrong, commander of Apollo 11, climb down from the lunar module spacecraft for a stroll on the moon. As his foot touched the lunar surface, he famously said "That's one small step for (a) man, one giant leap for mankind." This image is a black-and-white reproduction from the telecast, showing Armstrong stepping down from the lunar module's ladder.
(NASA Johnson Space Center)
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1969: Man on
the moon

Apollo 11 astronaut Buzz Aldrin, shown here, accompanied Armstrong for the famous walk on the moon. This iconic image is one of the few that shows Armstrong on the lunar surface - as seen in the reflection on the spacesuit's visor. The astronauts walked around on the lunar surface for about two and a half hours.
(NASA)
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1994: Looking
for ice

This mosaic image of the moon's southern polar region, made by the Clementine spacecraft in 1994, suggested that the region could harbor water ice within regions of its craters that are never lit by the sun. The water ice would be left over from impacting comets. Scientists have debated the evidence for and against water ice at the poles ever since the Clementine discovery. The current era of lunar exploration could resolve the debate. If water ice exists, it could help quench the thirst of future human colonists and be used to make fuel for rockets.
(NASA / JPL / USGS)
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2006: European moon probe crashes

On Sept. 3, 2006, the European Space Agency's SMART-1 spacecraft went out with a bang - a planned crash landing into a volcanic plain called the Lake of Excellence. The impact, shown here, was captured by the Canada-France-Hawaii Telescope. (The black lines are a processing error due to the brightness of the event.) The spacecraft was launched in 2003 primarily to test an ion propulsion system, which uses energy captured by the sun to produce a stream of charged particles. The slow-and-steady propulsion system may be used on future interplanetary missions.
(Christian Veillet / CFHT via AP)
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2007: China targets the moon

China made its first major strides in the lunar exploration game with the launch of the Chang'e 1 spacecraft in October 2007. The orbiter was sent to make a detailed, 3-D map of the moon's surface. Premier Wen Jiabao unveiled the first image at a ceremony in Beijing, shown here. Chang'e 1's 16-month mission ended with a controlled crash. The country reportedly plans to launch lunar rovers in 2010 and 2017, and a manned mission to the moon by 2020.
(Huang Jingwen / XINHUA NEWS AGENCY)
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2008: India joins the lunar club

The Indian Space Research Organization successfully launched its Chandrayaan 1 spacecraft on Oct. 22, 2008, for a mapping mission to the moon. A probe released from the mothership took this picture of the lunar surface during its descent to a planned crash landing at the south pole. The Indian space agency plans to use this and other data for a lunar rover mission in 2011 and, eventually, a manned mission.
(ISRO via EPA)
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2009: Japan orbiter watches eclipse

Some eclipse enthusiasts travel the globe to glimpse alignments of the sun, Earth and moon. Japan's Kaguya probe did them one better: It shot this sequence of a Feb. 10, 2009, eclipse from its lunar orbit. The image shows the view of the sun from the moon mostly covered by Earth. The "ring" appears dark at the bottom because it is obscured by the night-darkened limb of the moon. The Kaguya orbiter was launched in September 2007 to study the moon's origin and evolution. It made a controlled crash landing on the moon in June 2009.
(JAXA / NHK)
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2009: NASA goes back to the moon

On June 18, 2009, NASA launched two spacecraft to the moon to map its surface in unprecedented detail, scout for future landing sites, and smash probes into a permanently shaded crater in hopes of resolving a longstanding debate over whether such regions contain water ice. NASA's Lunar Reconnaissance Orbiter will orbit both poles for a year, and its mission could be extended to serve as a communications relay for future lunar missions. This is one of the first pictures sent back by the orbiter. LRO's sibling, the crater-smacking LCROSS probe, is due to impact the moon's south pole in October.
(NASA / GSFC / ASU)
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Editor's note:
This image contains graphic content that some viewers may find disturbing.